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New petrographic, geochemical, and isotopic (Sr, Nd, and δ18О) data on olivine and pyroxene phenocrysts provide constraints on the composition and crustal evolution of primary melts of Paleoproterozoic (2.40 Ga) picrodoleritic sills in the northwest Kola province, Fennoscandian Shield. The picrodolerites form differentiated sills with S-shaped compositional profiles. Their chilled margins comprise porphyritic picrodolerite (upper margin) and olivine gabbronorite (bottom) with olivine and clinopyroxene phenocrysts. Analysis of the available data allows us to recognize three main stages in the crystallization of mineral assemblages. The central parts of large (up to 2 mm) olivine phenocrysts (Ol–1–C) crystallized at the early stage. This olivine (Mg# 85–92) is enriched in Ni (from 2845 to 3419 ppm), has stable Ni/Mg ratio, low Ti, Mn and Co concentrations, and contains tiny (up to 10 μm) diopside–spinel dendritic lamella that probably originated due to the exsolution from high Ca- and Cr- primary magmatic olivine. All these features of Ol–1–C are typical of olivine from primitive picritic and komatiitic magmas (De Hoog et al., 2010; Asafov et al., 2018). Ol–1–C contains large (up to 0.25 mm) crystalline inclusions of high-Al enstatite (Mg# 80–88) and clinopyroxene (Mg# 82–90), occasionally in association with Ti-pargasite and chromian spinel (60.4 wt.% Al2O3). These inclusions are regarded as microxenoliths of wall rock that were captured by primary melt at depths more than 30 km and preserved due to the conservation in magmatic olivine. The second stage was responsible for the crystallization of Ol–1 rim (Ol–1–R), small (up to 0.3 mm) olivine (Ol–2, Mg# 76–85) grains, and central parts of large (up to 1.5 mm) clinopyroxene (Cpx–C) phenocrysts in the mid-crustal transitional magma chamber (at a depth of 15–20 km) at 1160–1350°C. At the third stage, Cpx–C phenocrysts were overgrown by low-Mg rims (Mg# 70–72) similar in composition to the groundmass clinopyroxene from chilled picrodolerite and gabbro-dolerite in the central parts of the sills. This stage likely completed the evolution of picrodoleritic magma and occurred in the upper crust at a depth of about 5 km. All stages of picrodoleritic magma crystallization were accompanied by contamination. Primary melts were contaminated by upper mantle and/or lower crust as recognized from xenocrystic inclusions in Ol–1–C. The second contamination stage is supported by the negative values of εNd(2.40) = –1.1 in clinopyroxene phenocrysts. At the third stage, contamination likely occurred in the upper crust when ascending melts filled gentle fractures. This caused vertical whole-rock Nd heterogeneity in the sills (Erofeeva et al., 2019), and difference in Nd isotopic composition of clinopyroxene phenocrysts and doleritic groundmass. It was also recognized that residual evolved melts are enriched in radiogenic strontium but have neodymium isotopic composition similar to other samples. It could be explained by the interaction of the melts with fluid formed via decomposition of biotite from surrounding gneisses under the effect of high-temperature melts. [ABSTRACT FROM AUTHOR]

1860-Ma within-plate mafic rocks of the Eastern Murman Sill Province were studied in the northern part of the Murmansk Craton, where they compose sheet bodies (sills) up to 100 m thick in Archean granitoids and gneisses. The sills have a simple inner structure, show no macroscopic signs of in situ differentiation even in large bodies, and consist of homogenous medium-grained poikilophitic dolerites weakly varying in composition. Minor variations in mineral and chemical composition of the rocks allow us to regard the mafic sills as I-shaped sills. Their homogeneity is likely caused by the rapid crystallization of aphyric magmas. The studied sills are close to N-MORB in major-element composition, but significantly differ in trace element signatures. The low Cr and Ni concentrations in the dolerites indicate the high degrees of fractional crystallization, while the low HREE and HFSE contents suggest that primary komatiite melts were formed through the high-degree melting of a depleted mantle source. The absence of phenocrysts in chilled dolerites prevented the use mineralogic and petrographic criteria for estimating the crystallization conditions in intermediate magma chambers. AlphaMELTS 1.9 and PRIMELT3 MEGA modelling results indicate that the primary melts could be formed through a high-degree melting of ascending high-temperature mantle plume, with subsequent contamination and fractional crystallization in mid-crustal intermediate magma chambers. The Nd and Sr isotopic composition of the dolerites and constituent minerals suggests that the crustal contamination included several stages. The first, most significant crustal contribution (up to 10%) was related to the contamination by Archean tonalites in the intermediate mid-crustal chambers. This provided negative εNd values and radiogenic Sr isotopic composition (Sri = 0.702–0.704) in the dolerites. The second stage included the upper-crustal contamination during lateral propagation of melts along gentle weakened zones, which resulted in the wide variations of Sr-Nd isotopic composition in rocks and minerals. The third stage was related to the contamination by 87Sr-enriched fluid that is supported by the sharp increase of Sri (up to 0.708) at constant Nd isotopic composition in gabbro-pegmatites. Such fluid could be formed via biotite dehydration caused by heating of host granites by large volume of emplaced mafic melts. The age, volume, and composition of the studied dolerites and similarity with coeval mafic rocks in the Superior Craton indicate that the I-shaped mafic sills in the Murmansk Craton could be formed on the periphery of the Circum Superior large igneous province. It suggests that the Murmansk Craton of the Fennoscandian Shield and the Superior Craton of the Canadian Shield at 1860 Ma were joined into a single consolidated lithospheric block that host within-plate mafic rocks and likely represented an early core of the Paleoproterozoic Nuna supercontinent. [ABSTRACT FROM AUTHOR]

The comprehensive geochemical and isotopic-geochronological study of Early Proterozoic volcanic rocks in structure of the Polmak–Pechenga–Imandra–Varzuga belt and dikes and sills of the Murmansk and Kola–Norwegian terranes is conducted. Abundant swarms of mafic dikes (2.06–1.86 Ga) are established in the northwestern frame of the belt, including swarms of metadolerites (2060 ± 6 Ma), ferropicrites and gabbronorites (1983 ± 5 Ma), and poikilophitic dolerites (1860 ± 4 Ma). The comparison of volcanic rocks of the Pechenga and Imandra–Varzuga structures shows asynchronous change in volcanism style, with a significant time lapse. The geochemical features of volcanic rocks of the Tominga Formation are typical of those of continental magmatism and can hardly be correlated with those of the Pilguyarvi Formation. According to isotopic-geochronological data, depleted mantle melts in the Pechenga and Imandra–Varzuga zones intruded at 2010–1970 and 1970–1980 Ma, respectively. The analysis of the conditions of formation of volcanic series shows that Neoarchean lithospheric mantle, which produced melts with low Zr/Nb ratios, was a source for primary melts of the Kuetsjarvi Formation of the Pechenga structure and their homologs of the Imandra–Varzuga structure. In contrast, the volcanic rocks of the Kolasjoki Formation, which were weakly contaminated with crustal material, and the related Ilmozero Formation, as well as the metadolerite dikes of the Kirkenes region, were sourced mostly from asthenosphere with separation of melt above the garnet stability depth. The formation of the volcanic rocks of the Pilguyarvi Formation is related, judging from the geochemical data, to two asthenospheric sources different in depth, which produced tholeiitic and ferropicritic melts. [ABSTRACT FROM AUTHOR]